Multiband r f switching device

ABSTRACT

A multiband transformation stage ( 14 ) comprising a common first signal port ( 20 ), a common second signal port ( 26 ) and a signal path ( 50 ) coupled between the first signal port ( 20 ) and the second signal port ( 26 ) is described. The signal path ( 50 ) is switchable between a first state with a first quarter-wavelength transformer characteristic for a first frequency band, a second state with a second quarter-wavelength transformer characteristic for a second frequency band and a third state with a transmission characteristic. The invention also relates to a multiband switching device comprising the multiband transformation stage ( 14 ) in combination with a low-power switching stage ( 16 ).

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field of the Invention

[0002] The invention relates to the field of radio frequency (R.F.)circuits and in particular to multiband R.F. circuits adapted to two ormore R.F. frequency bands like the frequency bands defined for theglobal system for mobile communication (GSM), e.g. 450 MHz (GSM 450),900 MHz (GSM 900), 1800 MHz (GSM 1800) and 1900 MHz (GSM 1900).

[0003] 2. Description of the Prior Art

[0004] R.F. circuits are utilized for a large variety of differentapplications. As an example, antenna switching circuits for mobiletelephones can be mentioned. Mobile telephones adapted to the timedivision multiple access (TDMA) mode, for example GSM systems, arecommonly using antenna switches for coupling an antenna port to either atransmitter path or a receiver path of the mobile telephone.

[0005] An antenna switch for a single frequency band and consistingessentially of two pin-diodes and a quarter-wavelength transformer isknown from WO 88/00760. The antenna switch is depicted in FIG. 1. In atransmit mode, both pin-diodes D1 and D2 are switched on. A transmitterport PTX is thus connected to an antenna port P_(ANT) via a firstpin-diode D1. A receiver port P_(RX) is connected to ground via a secondpin-diode D2 and the resulting short circuit at receiver port P_(RX) istransformed by the quarter-wavelength transformer to an open circuit atantenna port P_(ANT). Receiver port P_(RX) is thus isolated from antennaport P_(ANT). In a receive mode both pin-diodes D1 and D2 are switchedoff. In the receive mode, transmitter port P_(TX) is virtuallydisconnected from antenna port PANT and receiver port P_(RX) isconnected to antenna port P_(ANT) via the quarter-wavelengthtransformer. The switching state (on/off) of pin-diodes D1 and D2 iscontrolled by means of a control voltage V_(DC) applied to a controlport. Inductor L1 provides a DC path to pin-diodes D1 and D2 andresister R1 sets the DC current through pin-diodes D1 and D2.

[0006] For mobile telephones operable in a dual frequency band mode orin a triple frequency band mode the antenna switch depicted in FIG. 1has to be modified. In dual band applications like GSM 900/GSM 1800 orGSM 900/GSM 1900 for example a diplexer circuit may be inserted into thecommon antenna path. The diplexer circuit splits incoming antennasignals into high-band signals and low-band signals. The incominghigh-band signals and low-band signals are thereafter individuallyapplied to separate antenna switches. Thus a first antenna switch forhigh-band signals and a second antenna switch for low-band signals hasto be provided, each antenna switch further splitting the antenna pathinto a transmitter path and a receiver path. Triple-band applicationslike GSM 900/GSM 1800/GSM 1900 usually also utilize a single diplexercircuit for splitting the common antenna path into a low-band signalpath (GSM 900)/and a combined high-band signal path (GSM 1800/GSM 1900).

[0007] The use of a diplexer circuit for splitting a signal incident atthe antenna port into low-band and high-band signals leads to a rathercomplex circuit design. Therefore, antenna switches configured to alsoperform the signal splitting function of a diplexer circuit have beenproposed.

[0008] An antenna switch for coupling a single antenna to either one ofa first and a second receiver, operable at a first and a secondfrequency band, respectively, and a first and a second transmitter,operable to transmit at the first and the second frequency band,respectively, is known from DE 197 04 151. The antenna switch has amultiband transformation stage 100 as schematically depicted in FIG. 2.The multiband transformation stage 100 comprises a common signal input102, two separate signal outputs 104, 106, two quarter-wavelengthtransformers SL1, SL2 coupled in series and three switching elementsSE3, SE4, SE5.

[0009] The two quarter-wavelength transformers SL1, SL2 coupled inseries represent together a quarter-wavelength transmission line at afirst frequency band and each single quarter-wavelength transformer SL1,SL2 represents a quarter-wavelength transmission line for a secondfrequency band equaling approximately twice the first frequency band.

[0010] Signal input 102 of the multiband transformation stage 100 isusually coupled to an antenna and to a multiband transmitter switch forcoupling either a first transmitter operable in the first frequency bandor a second transmitter operable in the second frequency band to theantenna. First signal output 104 may be coupled to a first receiverreceiving in the first frequency band and second signal output 106 maybe coupled to a second receiver receiving in the second frequency band.

[0011] The multiband transformation stage 100 has four operationalstates. In a first operational state corresponding to transmission inthe first frequency band, switching elements SE3 and SE4 are switchedoff and switching element SE5 is switched on. The short circuit createdby switching element SE5 at a node 108 is transformed to an open circuitfor the first frequency band at signal input 102 of the multibandtransformation stage 100. In a second operational mode corresponding totransmission in the second frequency band, switching element SE3 isswitched on and switching elements SE4 and SE5 are switched off.Switching element SE3 thus creates a short circuit at a node 110. Thisshort circuit is transformed by the quarter-wavelength transmission lineSL1 to an open circuit for the second frequency band at signal input102. In a third operational state corresponding to receiving in thefirst frequency band, switching elements SE3, SE4 and SE5 are turnedoff. Consequently, first signal output 104 is coupled impedance-matchedvia the two quarter-wavelength transmission lines SL1, SL2 with signalinput 102. In a fourth operational state corresponding to receiving inthe second frequency band, switching element SE3 is turned off andswitching elements SE4, SE5 are turned on. This means that second signaloutput 106 is coupled impedance-matched via first quarter-wavelengthtransmission line SL1 with signal input 102. Further, the short circuitcreated by switching element SE5 is transformed by secondquarter-wavelength transmission line SL2, which has a quarter-wavelengthcharacteristic for the second frequency band, into an open circuit atsecond output port 106.

[0012] The fourth operational stage necessitates that the twoquarter-wavelength transmission lines SL1, SL2 have an identicaltransformation characteristic. This requirement, however, limits theapplicability of the multiband transformation stage 100 depicted in FIG.2 to the case where the first frequency band equals approximately halfthe second frequency band. A further disadvantage of the multibandtransformation stage 100 is the fact that in the fourth operationalstate, i.e. in high-band receive mode, two switching elements SE4, SE5are in an on state. This leads to a considerable current consumption inthe order of milliamperes and reduces the stand-by time ofbattery-powered devices. Moreover, the multiband transformation stage100 comprises altogether three switching elements SE3, SE4, SE5 whichhave to be biased. This requires a comparatively complex biasingnetwork. The biasing network becomes even more complex if the multibandtransformation stage 100 has to be adapted for triple-band applications.

[0013] Also, the multiband transformation stage 100 suffers from limitedisolation between signal input 102 which may be coupled to transmittersand signal outputs 104, 106 which may be coupled to receivers. Thismeans that terminations of signal outputs 104, 106 become relevant inthe first two operational states, i.e. in transmit modes. Terminationsof output ports 104, 106, however, are difficult to design due to theconstraints imposed by the receivers coupled to output ports 104, 106.

[0014] A further multiband switching device with a multibandtransformation stage is known from WO00/41326. The multiband switchingdevice is utilized for switching an antenna port between two poweramplifier ports and a receive port. The multiband transformation stagecomprises one or more diode switches positioned between a first one ofthe amplifier ports, a second one of the amplifier ports and ground. Oneor a plurality of frequency dependent isolation sections are positionedbetween the first amplifier port, the second amplifier port and thereceive port so that the diode switches are controlled using arespective control input for connecting either the first amplifier port,the second amplifier port or the receive port to the antenna port. Asfar as the low-power side of the multiband switching device known fromWO00/41326 is concerned, the transformation stage suffers from a lowflexibility.

[0015] There is, therefore, a need for a multiband switching devicewhich does not suffer from the limitations of the prior art switchingdevices.

SUMMARY OF THE INVENTION

[0016] The existing need is satisfied according to the invention by amultiband switching device comprising a multiband transformation stageand a low-power stage. The multiband transformation stage has a firstcommon signal port, a second common signal port and a signal pathcoupled between the first common signal port and the second commonsignal port, the signal path being switchable between a first state witha first quarter-wavelength transformer characteristic for a firstfrequency band, a second state with a second quarter-wavelengthtransformer characteristic for a second frequency band and a third statewith a transmission characteristic for at least the first frequency bandand the second frequency band. The low-power stage, which may berealized in the form of a low-power switch, has a first signal portcoupled to the second signal port of the multiband transformation stageand a plurality of second signal ports which may be coupled to the firstsignal port of the low-power stage.

[0017] The multiband switching device according to the invention can beused in all multiband environments that require a transformation stagethat enables coupling of an electrical component to an input/output portin a first mode (third state of the multiband transformation stage) anddecoupling of the electrical component from the input/output port in asecond mode (first state and second state of the multibandtransformation stage). Preferably, the multiband transformation stage isused to decouple in a transmit mode a multiband transmitter switch,which is coupled to an antenna port, from a multiband receiver switch ora multiband transmitter/receiver switch, which is coupled to the antennaport via the multiband transformation stage.

[0018] The multiband transformation stage may easily be adapted to morethan two different frequency bands, for example to triple-band orquadruple-band applications. In this case the signal path may beswitchable among further states, each further state corresponding to anindividual quarter-wavelength transmission characteristic for anindividual further frequency band. If frequency bands are only slightlyspaced apart, however, a single state may be allocated to such frequencybands since a state having a quarter-wavelength transmissioncharacteristic for one of these frequency bands will also have a fairlygood quarter-wavelength transmission characteristic for nearby furtherfrequency bands.

[0019] According to the invention, a signal fed in the third state intothe multiband transformation stage via one of its common signal ports istransferred to the other common signal port regardless of its frequency.If desired, individual signal paths for individual frequency bands maytherefore be selected only after the signal is output by the multibandtransformation stage. For example, one common signal port may be splitup into several individual ports. Since the signal path needs notnecessarily be selected within the multiband transformation stage thereare less constraints with respect to the construction of the multibandtransformation stage. This allows a less sophisticated and a moreflexible realization of the multiband transmission stage.

[0020] For example, the third state of the signal path can be realizedwithout the need to turn any switching elements on. The powerconsumption in the third state can thus be kept very low. Moreover, theuse of the multiband transformation stage is no longer limited tofrequency bands having a specific frequency ratio. Also, when splittingup the signal path after the signal has propagated through the multibandtransformation stage, specific termination ports having a predeterminedtermination impedance can be provided, thus enhancing isolation of themultiband transformation stage in the first and second state of thesignal path.

[0021] The switchable signal path of the multiband transformation stagecan be realized in various ways. According to a preferred embodiment,the signal path has two signal path portions coupled in series. Forexample, a first signal path portion is coupled between the first signalport of the mulitband transformation stage and a first node and a secondsignal path portion is coupled between the first node and the secondsignal port of the multiband transformation stage. Preferably, eachsignal path portion has a specific quarter-wavelength characteristic.The quarter-wavelength characteristics of the individual signal pathportions can be chosen such that the first signal path portion has aquarter-wavelength characteristic for the first frequency band and thatthe first signal path portion and the second signal path portiontogether have a quarter-wavelength characteristic for the secondfrequency band.

[0022] If the multiband transformation stage is used in a multibandenvironment in which a third frequency band appears which has a greaterdistance from the first and the second frequency bands, the signal pathmay comprise a third signal path portion. This third signal path portioncan be coupled in series with the first and the second signal pathportions and may have a quarter-wavelength characteristic which ischosen such that the three quarter-wavelength portions together have aquarter-wavelength characteristic for the third frequency band. Thisconcept can analogously be extended if four or more substantiallydifferent frequency bands are employed.

[0023] The switching of the signal path may be performed by means ofswitching elements which are preferably independently switchable fromeach other. One switching element may be provided for each signal pathportion. One switching element each may be coupled to one of the twoends of a respective signal path portion. The switching elements may bearranged such that they allow to selectively short-circuit therespective ends of the signal path portions. Due to thequarter-wavelength characteristic of each signal path portion the shortcircuit is transformed, for a specific frequency band, to an opencircuit at one of the signal ports of the multiband transformationstage. By appropriately switching the single switching elements thesignal path thus becomes tunable with respect to variousquarter-wavelength characteristics.

[0024] According to a preferred embodiment, the multiband transformationstage is constructed in multi-layer technology or with discretecomponents. Due to the fact that the multiband transformation stage isof comparatively low complexity, standardized low price multi-layertechnology can be used. Preferably, the multiband transformation stageis realized as an individual device which allows to insert the multibandtransformation stage in a modular manner in existing environments.

[0025] The multiband transformation stage according to the invention canadvantageously be employed for realizing multiband switching deviceslike multiband antenna switches. A multiband switching device maycomprise at least one of a high-power stage coupled to the first signalport of the multiband transformation stage and the low-power stagecoupled to the second signal port of the multiband transformation stage.Preferably, the multiband switching device further comprises a commonnode to which an input/output port, the high-power stage and the firstsignal port of the multiband transformation stage is coupled.

[0026] The multiband transformation stage may be used to decouple thehigh-power stage from the low-power stage. Also, the multibandtransformation stage can be used to appropriately terminate the commonnode of the high-power stage, the input/output port and the signal portsof the multiband transformation stage. Thus, a pre-defined impedance atthe common node can reproducibly be realized.

[0027] By means of the low-power stage (functioning as a low-powerswitch) the single signal path of the multiband transformation stage canselectively be connected to one of the plurality of second signal portsof the low-power stage. For example, the low-power switch may have anindividual signal input or output port for each frequency band. In thecase of mobile telephones operable for example in a GSM 900, GSM 1800and GSM 1900 mode, the low-power switch may thus comprise threecorresponding second signal ports configured as signal output ports. Thelow-power switch may have additional second signal input or output portsfor signals such as a low-power transmitter signal, a global positioningsystem (GPS) signal or a Bluetooth signal.

[0028] Also, the low-power stage can have one or more second signalports which are configured as auxiliary ports. The auxiliary ports canbe terminated with different pre-determined impedances. By coupling themultiband transformation stage by means of the low-power switch to apre-determined impedance a high isolation of the multibandtransformation stage can be achieved. According to a further option, theone or more auxiliary ports can serve as input ports for a DC voltage.The DC voltage can e.g. be used for controlling switching elements orother components of the multiband transformation stage.

[0029] Preferably, the low-power switch is realized as a microwavemonolithic integrated circuit (MMIC) device. Since the multibandtransformation stage ensures a good isolation between the high-powerstage and the low-power stage, it becomes possible to use low-power MMICdevices, i.e. standard MMIC devices already available from a largenumber of suppliers at a comparatively low price. MMIC low-powerswitches have the additional advantage that the number of second signalports of the low-power switch can easily be increased up to five ormore. The number of e.g. signal output ports is thus no longerrestricted by the design of the multiband transformation stage.Moreover, the power consumption of MMIC devices is comparatively low.Therefore, the overall power consumption of the multiband switchingdevice is also low, especially when the switching elements of themultiband transformation stage are switched off.

[0030] Modular MMIC low-power stages can advantageously be combined withmodular multiband transformation stages and high-power stagesconstructed in multi-layer technology or with discrete components. Themodular concept allows a multi-sourcing of the individual modular stagesfrom different suppliers and is thus suitable for very high volumeproducts. Moreover, the modular concept minimizes design risks since themodular stages of the multiband switching device can be split up andverified separately. Also, the modular concept leads to more flexibilityin printed circuit board (PCB) design due to the possibility ofsplitting of the individual modular stages.

[0031] The multiband switching device is preferably employed as anantenna switch in mobile telephones. The low-power switch can thus beconfigured as a multiband receiver switch. Also, the low-power switchmay be configured as a multiband transmitter/receiver switch providedthat the multiband transmitter/receiver switch is subjected to only lowtransmit powers. Thus, low-power transmit signals may be fed into theantenna switch via the low-power stage. The high-power stage cancomprise a multiband transmitter switch.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Further aspects and advantages of the invention will becomeapparent upon reading the following detailed description of a preferredembodiment of the invention and upon reference to the drawings, inwhich:

[0033]FIG. 1 is a schematic diagram of a prior art single band antennaswitch;

[0034]FIG. 2 is a schematic diagram of a prior art dual-band antennaswitch;

[0035]FIG. 3a is a schematic diagram of a triple-band antenna switchaccording to the invention;

[0036]FIG. 3b is a schematic diagram of a quadruple-band antenna switchaccording to the invention;

[0037]FIG. 4 is a schematic diagram of a basic simulation setup for thetriple-band antenna switch of FIG. 3;

[0038]FIG. 5 is a table showing simulation models and data sets used forthe simulation setup of FIG. 4; and

[0039]FIG. 6 shows the simulation results of the simulation setup ofFIG. 4.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0040] In FIG. 3a a schematic diagram of a first embodiment of amultiband switching device according to the invention in the form of atriple-band antenna switch 10 is illustrated. The antenna switch 10 ispart of a mobile telephone operable in three frequency bands inaccordance with GSM 900, GSM 1800 and GSM 1900.

[0041] The antenna switch 10 depicted in FIG. 3a has a modular structureand consists essentially of a high-power stage 12, a multibandtransformation stage 14 and a low-power stage 16. A signal output 18 ofthe high-power stage 12, a first signal port 20 of the multibandtransformation stage 14 and an input/output port configured as antennaport 22 are each coupled to a node 24. A second signal port 26 of themultiband transformation stage 14 is connected to a signal input port 28of the low-power stage 16.

[0042] The high-power stage 12 is constructed in multi-layer technologyand is used as a multiband transmitter switch. It comprises a firstsignal input 30 and a second signal input 32 coupled to respectivetransmitters not depicted in FIG. 3a. The first signal input 30 is usedas common GSM 1800/GSM 1900 signal input, i.e. as high-band signalinput. The second input 32 is used as GSM 900 signal input, i.e. aslow-band signal input. A first high-power signal path 34 is coupledbetween the high-band signal input 30 and a node 36. A second high-powersignal path 38 is coupled between the low-band signal input 32 and thenode 36. Each high-power signal path 34, 38 comprises a low-pass filter40, 42 followed by a switching element in the form of a pin-diode D3,D4. The low-pass filters 40, 42 reduce the level of spurious transmittersignals at harmonic frequencies. The high-power stage 12 furthercomprises an individual biasing network not depicted in FIG. 3 for eachpin-diode D3, D4. Each biasing network may be configured like thebiasing network depicted in FIG. 1.

[0043] The multiband transformation stage 14 has a single signal path 50which connects the first signal port 20 and the second signal port 26 ofthe multiband transformation stage 14. The signal path 50 is constitutedby two signal path portions in the form of a first transmission line T1and a second transmission line T2 coupled in series. The firsttransmission line T1 is configured to have approximately aquarter-wavelength charcteristic for the two frequency bands of 1800 MHzand 1900 MHz corresponding to GSM 1800 and GSM 1900. The secondtransmission line T2 is configured such that the two transmission linesT1 and T2 coupled in series have approximately a quarter-wavelengthcharacteristic for the frequency band of 900 MHz corresponding to GSM900.

[0044] The multiband transformation stage 14 further comprises twoswitching elements in the form of pin-diodes D1, D2. The two pin-diodesD1, D2 can be switched independently from each other by means ofindividual biasing networks not depicted in FIG. 3a. Each biasingnetwork may be configured like the biasing network depicted in FIG. 1and may comprise an inductor and a resistor. The first pin-diode D1 iscoupled between ground and a node 52 connecting the two transmissionlines T1, T2. The second pin-diode D2 is coupled between a node 54 andground. The node 54 is further coupled to the second signal port 26 ofthe multiband transformation stage 14 and an end of the secondtransmission line T2 which faces the low-power stage 16.

[0045] The multiband transformation stage 14 is also constructed inmulti-layer technology. In accordance with the modular aspect of theinvention, the high-power stage 12 and the multiband transformationstage 14 are realized as individual stages on different substrates.Alternatively, the high-power stage 12 and the multiband transformationstage 14 could be integrated on a single multi-layer substrate.

[0046] The low-power stage 16 is basically a receive switch matrix witha signal input port 28 coupled to the second signal port 26 of themultiband transformation stage 14, three signal output ports 56, 58, 60and an auxiliary port 62. The low-power stage 16 has a signal input 64for a control signal which specifies which of the ports 56, 58, 60, 62is to be coupled to the signal input port 28 of the low-power stage 16.The low-power stage 16 has one signal output port for each frequencyband. The signal output port 56 defines the 1900 MHz signal path and iscoupled to a 1900 MHz receiver. The signal output port 58 defines the1800 MHZ signal path and is coupled to a 1800 MHz receiver. Finally, thesignal output port 60 defines the 900 MHz signal path and is coupled toa 900 MHz receiver. The receivers are not depicted in FIG. 3a. Theauxiliary port 62 is terminated with a pre-defined fixed impedance. Thefunction of the auxiliary port will be described later in more detail.

[0047] Now, the different operational modes of the antenna switch 10 aredescribed with reference to the following table: mode state of signalpath 50 D1 D2 transmit GSM 900 1 OFF ON transmit GSM 1800/GSM 1900 2 ONOFF receive GSM 900/GSM 1800/ 3 OFF OFF GSM 1900

[0048] First, the high-band transmit mode, i.e. transmission in the 1800MHz band or 1900 MHz band, is described. In the high-band transmit modeeither a GSM 1800 transmitter signal or a GSM 1900 transmitter signal isapplied to the first signal input 30 of the high-power stage 12.Pin-diode D3 is switched on and pin-diode D4 is switched off.Consequently, either the GSM 1800 transmitter signal or the GSM 1900transmitter signal is fed to the antenna port 22. In the multibandtransformation stage 14 pin-diode D1 is switched on and pin-diode D2 isswitched off during high-band transmission. This corresponds to thefirst state of the signal path 50 in which the short circuit at node 52is transformed by the first transmission line T1, which has aquarter-wavelength transformer characteristic at high-band frequencies,to an open circuit at the first signal port 20 of the multibandtransformation stage 14. Consequently, the low-power stage 16 remainsisolated from the high power stage 12 and the antenna port 22.

[0049] In the low-band transmit mode a GSM 900 transmitter signal isapplied to the second signal input 32 of the high-power stage 12.Pin-diode D3 is switched off and pin-diode D4 is switched on.Consequently, the GSM 900 transmitter signal is fed to the antenna port22. In the multiband transformation stage 14 pin-diode D1 is switchedoff and pin-diode D2 is switched on in the low-band transmission mode.This corresponds to the second state of the signal path 50 of themultiband transformation stage 14 in which the signal path 50 has aquarter wavelength transformer characteristic for the 900 MHztransmitter signal. The short circuit at node 54 is transformed by thetwo transmission lines T1, T2 to an open circuit at the first signalport 20 of the multiband transformation stage 14. Consequently, thelow-power stage 16 is isolated from both the high-power stage 12 and theantenna port 22 in the low-band transmit mode.

[0050] The low-power stage 16 is configured as a GaAs MMIC receiverswitch matrix. Such GaAs MMIC devices usually generate spurious signalsat harmonic frequencies in response to high-power output signals intransmit modes. These spurious signals are generated internally andappear finally at the antenna port 22. However, no further low-passfiltering can be provided at the antenna port 22. Therefore, spurioussignals have to be kept below certain limits specified in e.g. the GSMstandard. In the antenna switch 10 according to FIG. 3a transmittersignals present at node 24 are sufficiently attenuated by the amount ofisolation provided by the multiband transformation stage 14 such thatspurious signal generation within the low-power stage 16 is kept smalleven without taking further measures.

[0051] In the high-band and low-band transmit modes the low-power stage16 is switched such that the signal output 26 of the multibandtransformation stage 14 is coupled to the auxiliary port 62 which isterminated with a specific impedance. Such a fixed termination isadvantageous because it has been found that stop-band attenuation of thelow-pass filters 40, 42 arranged within the high-power stage 12 iseffected by the impedance present at node 24. Best transmissionperformance is achieved when the system impedance of for example 50 Ohmis present at node 24. In the prior art depicted in FIG. 1, however, itwas observed that unused ports like the signal outputs 104, 106 coupledto both the signal input 102 and corresponding receivers exhibit avarying impedance in the transmit modes. This is due to the fact thatreceiver filters lead to a mismatch at the signal outputs 104, 106 inthe transmit mode. A varying impedance at the signal outputs 104, 106,however, may modify the impedance of the switching elements SE3, SE4,SE5 of the multiband transformation stage 100 such that no effectiveshort circuit is created. Thus no proper transformation to an opencircuit impedance at the signal input 102 of the multibandtransformation stage 100 can be achieved. This usually effects thematching of the respective transmitters as well as the performance oflow-pass filters within a high-power stage coupled to the signal input102.

[0052] This problem of prior art antenna switches is overcome by theimplementation of the auxiliary port 60 which allows a fixed terminationof the signal output 26 of the multiband transformation stage 14 intransmit modes. By activating the auxiliary port 60 the impedance atnode 24 can thus be kept constant. Any transmitter signal will thereforebe attenuated due to the isolation provided by the multibandtransformation stage 14 and additionally by the isolation of thelow-power stage 16 with activated auxiliary port 60. The maximum inputpower requirements of the receiver filters can be decreased accordingly.High-power receive saw filters are therefore no longer necessary. Thesize of saw filter structures can thus be reduced. Furthermore,advantages for multi-burst transmit modes as required by general packetradio systems (GPRS) arise.

[0053] Up to now the low-band and the high-band transmit mode have beenillustrated. Next, the receive mode will be described. The receive modecorresponds to the third state of the signal path 50 within themultiband transformation stage 14. In the receive mode, all pin-diodesD1, D2, D3, D4 are switched off. Therefore, power consumption of theantenna switch 10 is very low in the receive mode.

[0054] Since both pin-diodes D1, D2 of the multiband transformationstage 14 are switched off in the third state, the two transmission linesT1, T2 can be considered as a pure transmission line withoutquarter-wavelength transformer characteristic.

[0055] In the receive mode, one of the signal output ports 56, 58, 60,i.e. a respective receiver, is coupled to the antenna port 22 in animpedance-matched manner.

[0056] In FIG. 3b a schematic diagram of the second embodiment of amultiband switching device according to the invention in the form of aquadruple-band antenna switch 10 is illustrated. The antenna switch 10is part of a mobile telephone operable in four frequency bands inaccordance with GSM 450, GSM 900, GSM 1800 and GSM 1900. The antennaswitch 10 depicted in FIG. 3b has some similarities with the antennaswitch of FIG. 3a. The same reference numbers are thus used forcorresponding components.

[0057] Again, the antenna switch 10 depicted in FIG. 3b has a modularstructure and comprises a high-power stage 12, a multibandtransformation stage 14 and a low-power stage 16. The high-power stage16 is constructed in multi-layer technology and is used as a multibandtransmitter switch having a first signal input 30 coupled to a GSM 450transmitter and a second signal input 32 coupled to a GSM 900transmitter.

[0058] The multiband transformation stage 14 is constructed inmulti-layer technology. A first transmission line T1 of the multibandtransformation stage 14 is configured to have approximately aquarter-wavelength characteristic for the frequency band of 900 MHzcorresponding to GSM 900. A second transmission line D2 of the multibandtransformation stage 14 is configured such that the two transmissionlines T1 and T2 together have approximately a quarter-wavelengthcharacteristic for the frequency band of 450 MHz corresponding to GSM450.

[0059] The low-power stage 16 is configured as transmit/receive switchmatrix with a single signal input/output port 28 coupled to a secondsignal port 26 of the multiband transformation stage 14, four signaloutput ports 56, 58, 60, 66, a signal input port 68, an auxiliary port62 and a control signal input 64. The signal output ports 56, 58, 60, 66are coupled to a 1900 MHz receiver, a 1800 MHz receiver a 900 MHzreceiver and a 450 MHz receiver, respectively. The receivers are notdepicted in FIG. 3a. The auxiliary port 62 is terminated with apre-defined impedance.

[0060] The input port 68 of the low-power stage 16 is coupled to eithera GSM 1800 transmitter path or to a GSM 1900 transmitter path of atransmitter stage not depicted in FIG. 3b. The maximum transmit poweroccurring at the signal input 68 is 30 dBm. Thus, the low-power stage 16may be configured as a GaAs MMIC transmitter/receiver switch matrix.Usually, such GaAs MMIC devices can handle powers up to approximately 30dBm. Therefore, the value of 30 dBm can serve as a limit with respect tolow-power and high-power signals. In future, MMIC devices operable athigher powers will become available. Thus, the limit between low-powersignals and high-power signals may shift accordingly.

[0061] The operation of the antenna switch 10 depicted in FIG. 3b issimilar to the operation of the antenna switch of FIG. 3a. Therefore, adetailed description is omitted. The different operational modes of theantenna switch 10 depicted in FIG. 3b are shown in the following table:mode state of signal path 50 D1 D2 transmit GSM 450 1 OFF ON transmitGSM 900 2 ON OFF transmit GSM 1800/ 3 OFF OFF GSM 1900 receive GSM450/GSM 900/ 3 OFF OFF GSM 1800/GSM 1900

[0062] In FIG. 4 a simulation setup for the antenna switch 10 depictedin FIG. 3a is shown. The simulation is based on measured S-parameterdata of pin-diode BAR 63 and additional simulation models available inthe HPADS library as shown in FIG. 5. The simulation does not includethe low-power stage 16 for selectively coupling the signal output 26 ofthe multiband transformation stage 14 to individual receivers orauxiliary ports. Moreover, the low-pass filters 40, 42 of the high-powerstage 12 have also not been included into the simulation setup. Itshould be further noted that the simulation setup does not include anycomponents required for biasing the pin-diodes.

[0063] As can be seen in FIG. 4, inductors L3 and L4 are connected inparallel to pin-diodes D3 and D4 in order to improve the isolation tothe first and second signal inputs 30, 32 of the high-power stage. Inthe high-band transmit mode a series resonant circuit consisting of theparasitic inductance of pin-diode D1 and of capacitor C1 is transformedinto an open circuit at the antenna port 22. In the low-band transmitmode a short circuit created by the serious resonance circuit consistingof the parasitic inductance of the pin-diode D2 and of capacitor C2 isalso transformed into an open circuit at the antenna port 22. Theinsertion losses calculated based on the simulation model of FIG. 4 areshown in FIG. 6.

[0064] While a preferred embodiment of the invention has beenillustrated and described, it will be clear that the invention is notlimited in this regard. Numerous modifications, changes, variations,substitutions and equivalents will occur to those skilled in the artwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

1. A multiband switching device (10), comprising: a) a multibandtransformation stage (14) having: a common first signal port (20) and acommon second signal port (26); and a signal path (50) coupled betweenthe first signal port (20) and the second signal port (26), the signalpath (50) being switchable between a first state with a firstquarter-wavelength transformer characteristic for a first frequencyband, a second state with a second quarter-wavelength transformercharacteristic for a second frequency band and a third state with atransmission characteristic; and b) a low-power stage (16) coupled tothe multiband transformation stage (14), the low-power stage (16) havinga first signal port (28) coupled to the second signal port (26) of themultiband transformation stage (14); and a plurality of second signalports (56, 58, 60, 62, 66, 68) which can be coupled to the first signalport (28) of the low-power stage (16).
 2. The multiband switching deviceaccording to claim 1, wherein the signal path (50) has a first portion(T1) coupled between the first signal port (20) of the multibandtransformation stage (14) and a first node (52) and a second portion(T2) coupled between the first node (52) and the second signal port (26)of the multiband transformation stage (14), the first portion (T1)having a quarter-wavelength characteristic for the first frequency bandand the first portion (T1) and the second portion (T2) together having aquarter-wavelength characteristic for the second frequency band.
 3. Themultiband switching device according to claim 2, wherein the secondportion (T2) is coupled between the first node (52) and a second node(54) coupled to the second signal port (26) of the multibandtransformation stage (14), and wherein the multiband transformationstage (14) further comprises, for switching between the three states, afirst switching element (D1) coupled to the first node (52) and a secondswitching element (D2) coupled to the second node (54).
 4. The multibandswitching device according to one of claims 1 to 3, wherein at least themultiband transformation stage (14) is constructed in multi-layertechnology or with discrete components.
 5. The multiband switchingdevice according to one of claims 1 to 4, wherein the low-power stage(16) has at least one signal port (62) terminated with a pre-determinedimpedance.
 6. The multiband switching device according to one of claims1 to 5, wherein at least the low-power stage (16) is a MMIC device. 7.The multiband switching device according to one of claims 1 to 6,wherein the low-power stage is a multiband receiver switch (16) or amultiband transmitter/receiver switch.
 8. The multiband switching deviceaccording to one of claims 1 to 7, further comprising a high-power stage(12) coupled to the first signal port (20) of the multibandtransformation stage (14).
 9. The multiband switching device accordingto claim 8, further comprising an input/output port, preferably anantenna port (22), coupled to the high-power stage (12) and the firstsignal port (20) of the multiband transformation stage (14).
 10. Themultiband switching device according to claim 8 or 9, wherein thehigh-power stage is a multiband transmitter switch (12).
 11. Themultiband switching device according to one of claims 8 to 10, whereinthe high-power stage (12) is constructed in multi-layer technology orwith discrete components.
 12. A mobile telephone comprising themultiband switching device (10) according to one of claims 1 to 11, themultiband switching device (10) being configured as antenna switch. 13.A low-power switch (16) for a multiband switching device (10) preferablyaccording to one of claims 1 to 11, the low-power switch having a firstsignal port (28) and a plurality of second signal ports (56, 58, 60, 62,66, 68) which may be coupled to the first signal port (28), wherein atleast one signal port (62) is terminated with an impedance which ischosen such that a node (24) of the multiband switching device (10) isterminated in an impedance-matched manner.